630 Life in the Great Oceans [Feb. 7, 



retire towards the tropics, some of them continuing to occupy the 

 temperate regions by restricting the period of reproduction to the 

 summer season. Thus, in the tropical and coral-reef regions we find 

 to-day the remnants of a former universal shallow-water fauna, which 

 has been replaced in polar regions l)y animals from the mud-line 

 having mostly a direct development. This would explain the simi- 

 larity between the two polar faunas, the more numerous individuals 

 but relatively fewer species in the polar areas as compared with the 

 tropical area, and the resemblance between polar shallow-water 

 animals and deep-sea forms. On the other hand, if we assume an 

 ocean with a universal low temperature and universal cold-water 

 fauna and flora, then with heating at the equator the present tropical 

 faunas and floras would be evolved. 



The "Challenger" dredgings and trawlings at all depths indicate 

 a gradual decrease in the number of species with increasing depth, 

 the abundance of individuals in the shallower waters contrasting 

 strongly with the few individuals per species in depths beyond 

 1000 fathoms. The ratio of species to genera falls regularly on 

 proceeding into deeper and deeper water, which might be explained 

 by supposing the deep sea to have been peopled by continuoas 

 migrations from the mud-line at many periods and from many 

 regions. Trawlings in the deep water of the Southern Ocean yielded 

 a greater variety of animals than similar hauls in shallow water : 

 one haul in 1600 fathoms gave 200 individuals belonging to 

 81) species (excluding Protozoa), while eight hauls between 1260 and 

 2600 fathoms yielded 272 species — an average of 34 -species per haul. 



The majority of deep-sea species have apparently a restricted 

 distribution, and the idea that a universal and peculiar fauna of great 

 antiquity overspread the deep ocean-floor has not been supported by 

 deep-sea investigations. Some deep-water species are much larger 

 than their shallow-water allies, and others present archaic characters, 

 but the relict fauna from remote geological periods which some 

 naturalists hoped would be found to survive in the deep sea has yet 

 to be discovered. 



[J. M.] 



